1. Field of the Invention
The present invention relates to a nonlinear optical fiber, a nonlinear optical device employing the nonlinear optical fiber, and an optical processing apparatus employing the nonlinear optical fiber.
2. Description of the Related Art
In recent years, with an increase of the Internet traffic, a further increase of the speed and the capacity of the information communication using an optical fiber communication system has been highly demanded. In order to realize the high speed and the large capacity of the information communication, it is required to increase the speed of the optical signal processing technology and to broaden the bandwidth of the optical amplification technology, which is used in the optical fiber communication system.
As a conventional optical signal processing technology, it is common to convert an optical signal into an electrical signal, perform a signal processing on the electrical signal that is converted from the optical signal with an electrical device, and convert the electrical signal on which the signal processing is performed back into the optical signal. In this method, because the optical signal has to be once converted into the electrical signal, the speed of the signal processing is limited by a response speed of the electrical device. However, considering the recently demanded signal processing speed, the response speed of the conventional electrical device is reaching the end of its capability.
At the same time, a lot of attention is paid to an all-optical signal processing technology in which an optical signal is processed as it is. In the all-optical signal processing technology, the optical signal is directly processed with an optical device without being converted into an electrical signal. Because the response speed of the optical device is even higher than the response speed of the electrical device, the all-optical signal processing technology can expedite the increase of the signal processing speed.
An example of the optical device used in the all-optical signal processing technology is a nonlinear optical device that uses a nonlinear optical phenomenon occurred in an optical fiber that transmits the optical signal. The nonlinear optical device using the nonlinear optical phenomenon occurred in the optical fiber can process the optical signal in a high speed because the nonlinear optical phenomenon shown a high-speed response, and at the same time, can reduce loss of the optical signal because the optical fiber has a low transmission loss. For this reason, attention is particularly focused on the all-optical signal processing technology recently, and its application to an optical signal processing apparatus is presently being studied.
Examples of the nonlinear optical phenomenon include four-wave mixing (FWM), self-phase modulation (SPM), cross-phase modulation (XPM), stimulated Brillouin scattering (SBS), and stimulated Raman scattering (SRS). Among these nonlinear optical phenomena, the FWM is used in a wavelength converter, an optical parametric amplifier (OPA), and the like. Also, optical signal processing technologies such as a pulse compression, a waveform shaping, and the like using the SPM have already been reported (see, for example, Japanese Patent Application Laid-open Publication No. 2004-117590 and Japanese Patent Application Laid-open Publication No. 2005-301009).
In order to realize the nonlinear optical device using such nonlinear optical phenomena occurred in the optical fiber, it is essential to realize an optical fiber having a capability of generating the nonlinear optical phenomena with high efficiency, that is, a nonlinear optical fiber having high optical nonlinearity. A method of increasing the optical nonlinearity of the optical fiber is, for example, to decrease the effective core area Aeff of the optical fiber by increasing the relative refractive index difference between the core and the cladding. Examples of the nonlinear optical fiber having high optical nonlinearity are disclosed in, for example, Japanese Patent Application Laid-open Publication No. 2002-207136 and Japanese Patent Application Laid-open Publication No. 2003-177266.
In order to use the FWM or the SPM efficiently, it is necessary to use a nonlinear optical fiber not only having high optical nonlinearity but also having stable wavelength dispersion characteristics in a direction the optical signal propagates through the optical fiber, i.e., the longitudinal direction of the optical fiber. Especially when using the FWM, it is important that the absolute value of the wavelength dispersion in the longitudinal direction of the nonlinear optical fiber should be small and stable to keep the efficiency of generating the FWM over a long distance. The wavelength dispersion characteristics of the optical fiber are mainly determined by a structure of the core and the cladding and a refractive index profile of the optical fiber.
However, in a conventional optical fiber, because the diameter of the core varies along the longitudinal direction of the optical fiber due to a fluctuation of the manufacturing conditions at the time of manufacturing the optical fiber, it is difficult to manufacture an optical fiber having stable wavelength dispersion characteristics in the longitudinal direction of the optical fiber. In particular, in nonlinear optical fibers having the refractive index profiles disclosed in Japanese Patent Application Laid-open Publication No. 2002-207136 and Japanese Patent Application Laid-open Publication No. 2003-177266, there is a large change in the wavelength dispersion with a change of the diameter of the core, although they have high optical nonlinearity. Therefore, it is difficult to manufacture a nonlinear optical fiber having stable wavelength dispersion characteristics in the longitudinal direction of the fiber with the refractive index profiles disclosed in Japanese Patent Application Laid-open Publication No. 2002-207136 and Japanese Patent Application Laid-open Publication No. 2003-177266.